The present invention relates generally to wastewater treatment, and in particular to the on-site treatment and purification of marine wastewater. One or more embodiments of the present invention relate to methods and systems for treating wastewater onboard a marine vessel and/or stationary offshore platforms.
In one embodiment of the invention, a method for treating wastewater comprises pumping wastewater slurry into a wastewater collection tank. The wastewater comprises raw sewage, black water, gray water, galley waste and combinations thereof. The wastewater slurry further comprises suspended solid particles, organic and inorganic matter, bacteria and entrained gas. A wastewater level sensor installed on the wastewater collection tank monitors the wastewater level in the wastewater collection tank. When the wastewater slurry reaches a predetermined upper threshold level, the sensor triggers automatic commencement of operation. The pumping of the wastewater slurry may be stopped when the wastewater level falls below a predetermined lower threshold level. The wastewater slurry is routed by a macerator pump for maceration of the suspended solid particles. During maceration, the solid particles are finely ground, thereby resulting in smaller sized particles occupying a larger surface area. A stream of macerated slurry may be diverted back to the wastewater collection tank. The remaining macerated slurry is piped to an electrolytic cell. The electrolytic cell oxidizes and disinfects the macerated slurry using a controlled volume of seawater or brine. Since the finely macerated particles occupy a larger surface area, oxidation and disinfection of the macerated slurry in the electrolytic cell is significantly improved. A defoaming agent is added to the oxidized and disinfected slurry prior to piping it to an electrocoagulation cell. The disinfected suspended solids may be agglomerated or flocculated in the electrocoagulation cell. The flocculated slurry is routed to a primary settling tank for separation of a floc-containing sludge and a substantially clarified supernatant. The substantially clarified supernatant is piped to a secondary clarifying tank to facilitate further separation of the sludge and the substantially clarified supernatant. The sludge from the settling and clarifying tanks is precipitated at the bottom of the tanks and is discharged. The turbidity level of the discharged sludge is continually monitored. When the turbidity level equals a pre-determined low value, the sludge discharge is stopped by automatically closing the valves on the sludge discharge pipe. The substantially clarified supernatant may discharged as a treated effluent.
The macerated slurry stream diverted back to the wastewater collection tank may be mixed with the wastewater slurry in the collection tank. This maintains a homogeneous blend within the wastewater collection tank. In one embodiment, a mixing pump may be positioned adjacent the macerator pump to continuously mix and recirculate the macerated slurry stream with the wastewater slurry in the wastewater collection tank.
A controlled volume of seawater may be mixed with the macerated slurry as it enters the electrolytic cell. The volume of seawater introduced may be dependent on the specific treatment capacity of the marine wastewater treatment system. The macerated slurry may be oxidized and disinfected by an electrochemical reaction occurring inside the electrolytic cell. In one embodiment of the invention, the macerated slurry may be contacted with an oxidizing agent within the electrolytic cell.
The oxidized and disinfected slurry is passed into an electrocoagulation cell for agglomerating the macerated solids and other suspended solids. The electrocoagulation cell may enhance the disinfected wastewater stream with metal particles that serve as nucleation sites forming a flocculation with organic matter. The electrodes in the electrocoagulation cell may get coated with the solid particles and floc with continual use. In one embodiment of the invention, the electrocoagulation cell is periodically subjected to an automated air and water purge. The purge flushes the coated particulate contaminants from the electrodes. The purge contents are piped to the primary settling tank.
The substantially clarified supernatant may be treated with one or more chemicals prior to discharge to neutralize residual chlorine to less than 0.5 mg/L. In one embodiment of the invention, an optimal amount of a reducing agent is injected into the substantially clarified supernatant using a metering pump. The reducing agent may be selected from the group consisting of sodium bisulfite, sodium sulfite, sodium thiosulfate and sulfur dioxide.
In one or more embodiments of the invention, the discharged effluent may comprise less than 25 mg/L Biological Oxygen Demand (BOD), less than 35 mg/L Total Suspended Solids (TSS), less than 120 mg/L Chemical Oxygen Demand (COD) and less than 100 cfu/100 ml coli form.
In another embodiment of the invention, the flocculated slurry from the electrocoagulation cell and sludge and the particulate contaminants dislodged during the automated air and water purge are piped to a degasification chamber. The gases produced during the electrolysis reaction and other residual gases emitted from the slurry are diluted with ambient air and vented to the atmosphere. An electric air blower may be used to force the ambient air into the vent lines.
In another embodiment of the invention, the flocculated slurry exiting the electrocoagulation cell may be discharged into a polymerization tank. One or more cationic polymers may be introduced in the flocculated slurry to form polymerized agglomerated solid clusters. The polymerized agglomerated solid clusters may be filtered using a filtration unit.
In another embodiment of the invention, a system for treating wastewater comprises a wastewater collection tank, a macerator pump capable of grinding solids suspended in the wastewater, a mixing pump adjacent the macerator pump, an electrolytic cell, the electrolytic cell comprising a reaction chamber, an anode disposed within the reaction chamber and a cathode disposed within the reaction chamber, and means for providing power to the electrolytic cell, an electrocoagulation cell that is in fluid communication with the electrolytic cell, a settling tank adjacent the electrocoagulation cell, a clarifying tank connected to the settling tank, a turbidimeter for detecting turbidity levels of the discharged sludge, a dechlorination unit comprising a chemical injection pump or metering pump, and an effluent discharge pump. In one embodiment of the invention, the settling tank is connected to a degasification chamber. The degasification chamber comprises an electric blower and venting means to permit release of diluted gases produced during the electrolysis. In one embodiment of the invention, an optional sludge collection tank is positioned beneath the settling and clarifying tanks.
In one embodiment of the invention, the wastewater treatment system comprises a rigid base frame, wherein the rigid base frame is configured and disposed to carry the weight of the wastewater treatment system. In another embodiment of the invention, the wastewater treatment system comprises an air and water purger connected to the electrocoagulation cell. In yet another embodiment of the invention, a seawater supply source is connected the electrolytic cell.